You need to use Icon2incs. If you drag what ever icon you would like to use into that program, it will create a proper incs file for you to use. Then right-click Transmission, and select show package contents. Then open the contents folder, then the resources folder. There you will see the two icons that Transmission uses. rename them and copy your new icons into that folder. Rename the new icons to the same name the old icons used. Done....